Chicken globin gene transcription is cell lineage specific during the time of the switch

Posttranscriptional silencing of embryonic globin gene expression occurs during hemoglobin switching in chickens [Landes, G.M., Villeponteau, B., Pribyl, T.M., & Martinson, H.G. (1982) J. Biol. Chem. 257, 11008-11014]. Here we use Percoll density gradients to fractionate the red blood cells of 5-9-day embryos in order to determine the cellular source and the timing of this posttranscriptional process. By means of nuclear "run-on" transcription in vitro we show that it is within mature primitive cells that production of embryonic globin mRNA is terminated posttranscriptionally. In contrast, young definitive cells produce little (or no) embryonic globin mRNA because of regulation at the transcriptional level. Thus the lineage specificity of embryonic and adult globin gene expression is determined transcriptionally, and the post-transcriptional process described by Landes et al. is a property of the senescing primitive cells, not a mechanism operative in the hemoglobin switch. This conclusion is supported by [3H]leucine incorporation experiments on Percoll-fractionated cells which reveal no posttranscriptional silencing of the embryonic genes during the early stages of the switch. In the course of our studies we have noticed a strong transcriptional pause near the second exon of the globin genes which is induced by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) and which resembles a natural pause near that position.